A known electromagnetic switch for the on-off control of a starter motor for an internal combustion engine comprises an electric contact set which is actuated in synchronism with a reciprocating movement of an electromagnetic plunger for axially shifting a pinion into and out of mesh with a ring gear of an internal combustion engine.
In such an electromagnetic switch, a central rod slidably received in a stationary core is driven axially by the plunger which is magnetically attracted to the stationary core against the spring force of a return spring, and thereby moves a planar moveable contact member carried by the central rod into and out of engagement with a pair of fixed contact members to selectively establish an electroconductive path between a battery and a starter motor. To maintain the contact pressure between the movable contact member and the fixed contact members, the moveable contact member is elastically supported by the rod.
The rod is also provided with a return spring interposed between a spring holder fixedly secured to the rod and an inner surface of a casing secured to the stationary core so that the rod may be returned to the original position when the electromagnetic coil of the electromagnetic plunger is deenergized.
When there is any positional deviations between the fixed contact members, the contact pressure between the moveable contact member and the fixed contact members may become unevenly distributed, and chattering may be caused in the action of the moveable contact member in coming into contact with the fixed contact members. Conversely, if a thermal fusion takes place between the moveable contact member and the fixed contact members, a substantial amount of force may become necessary in removing the moveable contact member away from the fixed contact members. To avoid any problem associated with the difficulty in removing the moveable contact member away from the fixed contact member, the spring constant of the return spring must be increased, but this in turn necessitates the magnitude of the magnetic attraction to be increased, and this leads to a need for an over-sized electromagnetic coil.
Obviously, the present invention can be applied not only to the case where the electric contact is established by the energization of the electromagnet but also to the case where the electric contact is established by the spring force of the return spring.